Field of the Invention
The present invention relates to a finite impulse response (referred to hereinafter as FIR) digital filter device, and particularly to a FIR digital filter for use in boosting mid-spatial frequencies as opposed to the frequencies near the sensor resolution limit.
It is common in video systems to provide a digital enhancement filter in the electronics in order to improve the resolution or sharpness of the final image. This filter is usually implemented as a FIR filter and operates on the green signal after it has been interpolated. The green channel is usually the only colour that is enhanced, since most of the video detail signal is derived from this channel. One- and two-dimensional versions are available, the two-dimensional filters acting only on an individual field of the sensor.
Filters are generally designed to boost the high-spatial frequencies in the image. These frequencies are near the Nyquist frequency of the sensor, which equals xc2xdx, where xs is the horizontal pixel spacing. The process operates on the interpolated G signal from one, three or more lines or rows of pixels. The filtering is done on the intensity signals rather than the log intensity signals. In the case of a one-dimensional filtering process, a one-dimensional FIR filter is applied to the video signal through pixel delays and adds to provide a horizontal detail signal at each pixel:
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The summation of these values is zero so that the filter has no response to low frequency or constant scenes. The separation of these filter entries is one pixel. This horizontal signal is added to the original video signal to improve the image sharpness. The amount of the boost can be varied by changing the relative amount of detail signal that is added to the original signal.
In some applications it is important that the sharpness or modulation of the mid-spatial frequencies be enhanced instead of the spatial frequencies near the Nyquist resolution limit. In these cases, the above boost filter is ineffective and may actually serve to enhance the noise in the image. This situation is especially true if the video signal shows column-to-column variations as are often seen on colour systems with green checkerboard colour filter array patterns. Looking at the existing boost filter, the detail signal is derived from pixel-to-pixel variations and these would tend to correspond to spatial frequencies that are near the Nyquist frequency.
From the foregoing it may be appreciated that a need has arisen for a FIR digital filter which improves the modulation of the image from a video sensor at lower spatial frequencies.
An object of the present invention is to provide a digital FIR filter designed to boost mid-spatial frequencies.
Another object of the present invention is to provide a digital FIR filter designed to enhance the sharpness or modulation of the spatial frequencies at about half the Nyquist frequency.
In accordance with one aspect of the present invention, there is provided a finite impulse response digital filter device for processing an input signal and improving detail components of an image signal, the device being part of a signal processing network, said device comprising:
(a) digital filter means for digital filtering the input signal so as to provide a boost signal; and
(b) circuit means for adding the boost signal to the input signal so as to improve the sharpness or modulation of the mid-spatial frequencies of the image signal coming from a video sensor.
The present invention therefore provides for a digital FIR filter designed to improve the sharpness or modulation of the mid-spatial frequencies in the video signal and in the resulting image. The preferred implementation of this invention utilizes a FIR filter that operates on every other pixel, thus providing an easy to implement device.
Further objects and advantages of the present invention will he apparent from the following description, reference being made to the accompanying drawings wherein preferred embodiments of the invention are clearly shown.